The equilibrium solubility of the quaternary system CsCl-LuCl 3 -HCl-H 2 O was determined at 298.15 K, and the corresponding equilibrium diagram was constructed. The quaternary system is complicated with five equilibrium solid phases, CsCl, Cs 4 LuCl 7 ‚5H 2 O (4:1 type), Cs 9 Lu 5 Cl 24 ‚29H 2 O (9:5 type), Cs 2 LuCl 5 ‚2H 2 O (2:1 type), and LuCl 3 ‚6H 2 O, of which the new compounds Cs 4 LuCl 7 ‚5H 2 O and Cs 9 Lu 5 Cl 24 ‚29H 2 O were found to be congruently soluble and the new compound Cs 2 LuCl 5 ‚2H 2 O was found to be incongruently soluble in the system. The three new compounds obtained were identified and characterized by the methods of X-ray diffraction, thermogravimetry, and differential thermogravimetry. The standard molar enthalpy of solution of Cs 4 LuCl 7 ‚5H 2 O, Cs 9 Lu 5 Cl 24 ‚29H 2 O, and Cs 2 LuCl 5 ‚2H 2 O in deionized water was measured to be (15.6 ( 0.3) kJ‚mol -1 , (-78.8 ( 0.6) kJ‚mol -1 , and (-62.5 ( 0.5) kJ‚mol -1 , respectively, by heat conduction microcalorimetry. Their standard molar enthalpies of formation were calculated to be (-4313.0 ( 2.1) kJ‚mol -1 , (-17871.6 ( 5.3) kJ‚mol -1 , and (-2526.5 ( 1.2) kJ‚mol -1 , respectively.